Saturday, December 20, 2014

Howard Pass National Wind Chill Record

** This is a working draft as the National Park Service may have suggested edits or additions **

On February 14, 2014, a remarkable event occurred along the northern slopes of the Brooks Range above the Arctic Circle. Namely, a purported wind chill of -97°F was observed by an automated wind instrument located at Howard Pass operated by the National Park Service. The 2014 event was described and modeled in great detail by Richard here and here. I encourage you to read those posts.

The station is part of the Remote Automatic Weather Stations (RAWS) network and is referred to as Howard Pass RAWS. This event was well publicized by the National Weather Service (NWS) as a possible state and national record. Both Alaska and national newspapers carried the story and for a few days it generated a fair amount of discussion nationally. Unbeknownst to all but a handful of people at the time was an even lower wind chill recorded at the same station one year earlier. This earlier event was unknown at the time of occurrence due to the malfunctioning of transmission equipment. The onsite data logger continued to function during the event and when the data were retrieved several months later, the dramatic observations were revealed.

At 10 p.m. on February 21, 2013, a 2-meter air temperature of -47.5°F was observed along with a sustained wind of 53.7 miles per hour at the Howard Pass RAWS weather station. The combination of these two meteorological conditions produced a staggering wind chill of -99.8°F! The following sections of this report describe the setting where the station is located and the meteorological conditions present at the time.

Howard Pass

The Howard Pass area is currently uninhabited but indigenous people have occupied the region for thousands of years. The pass is an important caribou migration corridor and hence historic and prehistoric hunters utilized this location for subsistence and other traditional activities. The National Park Service participates in a program to establish and maintain weather stations in remote sections of Alaska on their land holdings. Howard Pass is within the six million acre Noatak National Preserve, created in 1980 by the Alaska National Interest Lands Conservation Act (ANILCA). The Noatak National Preserve is managed by the National Park Service and part of the justification of the Preserve's establishment was to promote scientific research in the area (ANILCA Section 102 (8)(a)). The National Park Service decided in 2007 to place an automated station at Howard Pass for the following reasons: 1) there are no nearby stations, 2) it is a major caribou migratory corridor, and 3) there are significant archaeological resources in the pass. This site location is also more accessible by helicopter than the pass itself due to the boggy nature of the pass.

The Howard Pass RAWS station is located at latitude 68.156°N, longitude 156.895°W, and at an elevation of 2,062 feet in the northwestern Brooks Range of Alaska. The station sits near the top of a hill on the eastern flank of the pass approximately 300-400 feet above the pass level and is approximately 12 miles south of the northern limit of the Brooks Range and 85 miles north of the southern limit. The station was placed above the pass level to prevent migrating caribou from damaging the station. Figure 1 shows a map of the station’s location and Figure 2 shows an exaggerated relief map of the area. Figure 3 shows a picture of the station.

The station’s location makes it extremely susceptible to
both low temperatures and strong winds. Each of the winter months has an
average temperature between +5°F and -10°F and an average wind speed between 17
and 25 miles per hour. Figure 4 shows the monthly averages between 2011 and 2014.
Just looking at the month of February, the combination of temperature and wind
produces a month-long average wind chill of -36°F!

Figure 4. Monthly average temperature and wind at the Howard Pass RAWS station (2011-2014).

All of northern Alaska is characterized by long, severe winter conditions. Snow lies on the ground from October through May and temperatures are below zero for nearly one third of the year. The passes of the Brooks Range are also subjected to intense winds that are funneled through topographically constrained areas. The cold, dense nature of the airmass facilitates a semi-permanent inversion layer that provides a vertical constraint on the wind movement. The combination of the horizontal constriction due to topography and vertical restrictions due to an inversion layer causes tremendous winds to occur when very low temperatures are present and a north-to-south pressure gradient exists. This setup frequently produces exceptionally low wind chill values.

February 21, 2013

If you only had a satellite image to look at, nothing about February 21, 2013, would stand out. The MODIS satellite image from this date (see Figure 5) reveals nothing out of the ordinary. The entire region is snow covered and few, if any, clouds are present.

What does not show up in the MODIS image is the dynamic nature of the atmosphere. The map shown in Figure 6 is a plot of surface weather conditions at 9 p.m. on February 21, 2013, from the Weather Prediction Center (WPC). A strong area of high pressure is centered far to the north of Alaska while a strong low pressure system was present in the Gulf of Alaska. These two features generated a tight pressure gradient resulting in strong winds across the northern one third of the state. Also evident are low temperatures and strong northeast winds.

Figure 7 is a map of temperature and wind chill values at 10 p.m. on February 21, 2013. The map illustrates the intensity of the cold air and the severity of the wind chill at the peak of the event. Many stations reported wind chills in the -40°s, -50°s, and -60°s. Anaktuvuk Pass, in the central Brooks Range, reported a wind chill of nearly -70°F. At this same time, the Howard Pass RAWS station reported a wind chill of -99.8°F.

We should note that this was not a short-lived event. For the 9-day period between February 15, 2013, and February 24, 2013, the 2-meter temperature at Howard pass averaged -32.6°F and the 3-meter wind speed averaged 33.1 miles per hour (mph). The average wind chill during those 9 days was -71°F. Figure 8 shows the temperature and wind chill for Howard Pass RAWS during the time period described above.

Figure 8. Hourly temperature and wind chill values observed at the Howard Pass RAWS station between February 16, 2013, and February 24, 2013.

For nearly 48 hours, the wind chill at Howard Pass RAWS oscillated between -90°F and -100°F – with the peak value of -99.8°F reported at 10 p.m. on February 21st. During those 48 hours, the average temperature was -45.1°F and the average wind speed was 49.1 mph. We should note that Howard Pass has reported winds in excess of 80 mph on numerous occasions. The event of February 2014 that produced a wind chill of -97°F included a stretch of time with temperatures in the -30°s and sustained winds up to 90 mph. Interestingly, once wind speeds exceed 50 mph, wind chill values do not change very much. Figure 9 shows the conditions observed at Howard Pass during the low wind chill event of February 2014 that produced a minimum reading of -97°F.

Figure 9. Hourly temperature and wind chill values observed at the Howard Pass RAWS station between February 11, 2014, and February 15, 2014.

Station Equipment

Is a -99.8°F wind chill even possible or should we immediately treat the observations as suspicious and figure out what went wrong? Let us first look at the equipment present at the site (also see photograph in Figure 3).The following bulleted points were relayed from National Park Service staff:

Station installation date: 7/13/2011. Station blew over on 8/16/2011 and was not repaired until 7/16/2012. Most sensors were not operating correctly August 2011 - July 2012.

There are two air temperature sensors at the station. The primary sensor is a YSI ThermX and the backup sensor is a Vaisala HMP155. For the period 2011 through July 2013 the backup sensor (Vaisala) was transmitting instead of the primary sensor (YSI).

The transmitting air temperature sensor (Vaisala) failed on August 3, 2012 and was not repaired until summer 2013. During the summer 2013 field visit, the primary sensor data were recovered from the data logger.

Additional air temperature sensor information:

Model YSI 44211

Height: 2.0 meters

Linear Range: -55°C to +85°C

+/- 0.18° at -55°C

+/- 0.02°C at +85°C

Additional wind sensor information:

Model RM Young 05103

Height 3.0 meters

The station is clearly located in an exposed area. Strong winds have disabled the station several times in the past and have sand-blasted the equipment. This alone lends credence to extreme wind observations. Nothing in the list of station equipment though stands out as a disqualifier for any observations.

Event Reconstruction

To look at the plausibility of this event, we conducted a model simulation using the Weather Research and Forecasting (WRF) model for the time period in question. The model was initialized using February 21, 2013, 0.5°GFS data from 00:00 UTC. A recursive, nested grid structure of 27-9-3-1 kilometers were modeled over a 96-hour period. The model uses 30 arc-second topography – equivalent to 1 square kilometer grid cells – to represent the terrain.

The model showed very cold temperatures, strong winds, and very low wind chills during the time period in question. Figure 10 shows the estimated temperatures for the portion of the model area centered on Howard Pass. Cold air is clearly advected part of the way through the pass. The temperature only drops about 5°F in the 12 miles from the northern entrance of the pass until the weather station.

Figure 11 shows the modeled sustained wind speed. The wind was only estimated to be around 35 miles per hour through the pass. Note how tightly packed together the wind streamlines are. There is a rather large discrepancy between the observed wind speeds and the modeled wind speeds. The hill where the station resides is not accounted for in the rather coarse (1 km) topographic dataset that the model uses. Interestingly, the model predicts an increase in winds of 5-10 miles per hour 100-200 meters above the pass level. Since the hill where the station is located is about 100-200 meters above the pass level, this is a reasonable, but still too low, proxy estimate for the wind speed at the station location. Figure 12 shows the modeled vertical wind profile for the station’s location.

Finally, Figure 13 shows the wind chills for the portion of the model area centered on Howard Pass. There are several black regions at the northern entrance of Howard Pass. Those grid cells represent wind chill values between -90°F and -95°F. These are the lowest values for any portion of the model domains. At the location of Howard Pass RAWS, the estimated wind chill was -85.4°F. As noted in the earlier paragraph, the model’s elevation dataset is not detailed enough to represent this area of complex topography.

Overall the model did a reasonable job of representing this event with the exception of under forecasting the winds. Table 1 summarizes the difference between the actual observations and the model estimates. We suspect that with a higher resolution topographic dataset, the modeled wind speeds would approach the observed wind measurements.

Table 1. Comparison of modeled and observed conditions on February 21, 2013, at 10 p.m. Alaska Standard Time (AST).

Wind Chill Record

Is the -99.8°F wind chill a U.S. record? The -97°F wind chill from February 2014 was informally determined to be a statewide and a national record. Unfortunately there is no database of lowest wind chill readings. The National Climate Data Center (NCDC) does not track low wind chills as part of their state or national records database.

Back in 2001, the National Weather Service (NWS) adopted a new formula for computing wind chill values. Prior to 2001, wind chill values as low as -120°F were observed at several location. However, the “new” formula generally has higher (warmer) wind chill readings that the “old” formula given the same temperature and wind speeds. When those older observations were evaluated with the new formula, they all came back in the -90°s range.

So what is the lowest reading and what criteria are used? Would you believe that the previous low wind chill reading in Alaska was also -99.8°F? The city of McGrath, Alaska, reported an air temperature of -72°F and a wind speed of 7 mph on January 27, 1989. Plugging these numbers into the wind chill formula gives us the -99.8°F value. Figure 14 shows the minimum wind chills in Alaska during the great January 1989 cold snap. All values were rounded to whole numbers – hence the -100°F value shown for McGrath. Figure 15 shows the hourly observations at McGrath on January 27 and January 28, 1989.

Figure 14. Minimum wind chills in January 1989 for all stations in Alaska that reported hourly observations using. For some stations, many observations are missing so a few data point should be considered suspect.

Figure 15. Hourly temperature and wind speed for McGrath, Alaska, on January 27 and January 28, 1989. Both the new and old wind chill values are displayed. The -99.8°F (-100°F) value using the new formula is shown.

The McGrath number comes with a qualifier though. With a wind of only 7 mph, should it even count? The Alaska NWS offices do not even issue Wind Chill Advisories or Wind Chill Warnings unless the sustained winds are, or are forecasted to be, 15 mph or greater for at least three hours. Maps of all Alaska advisory criteria can be found here. So, can a wind chill be a record if it wouldn't even qualify for a Wind Chill Advisory?

Several NWS offices have lists of statewide wind chill records. When the Minnesota Climatological Working Group discussed extreme wind chills, several examples they gave used wind speeds of 6 or 7 mph. The Montana Climate Atlas uses 10 mph as a threshold for their monthly probability maps. The lowest statewide wind chill value in their atlas was -80.9°F.

The NWS office in Lacross, Wisconsin, has a climatology of wind chills in the Northern U.S. (Lower 48). They use a 10 mph filter to develop their probability maps. In their report, they state, "[t]his speed was chosen as it is the minimum threshold currently used throughout most of the NWS for the issuance of Wind Chill Advisories or Warnings." Since their report is looking at climatological probabilities, it makes sense that they have a wind speed cutoff. Their report does not contain a list of extreme wind chills.

Given that no wind chill values in the Northern states claim to even approach -100°F, it seems unlikely that any place in the Lower48 has approached this value using the 2001 formula.

Another consideration is the nature of the formula itself and whether it is even applicable at such extreme values. It is worth noting that the original research that went into developing the new wind chill formula did not use air temperatures lower than -40°F and that the fitting of a formula to the observational data is the only reason it can be extended backward. Nevertheless, one of the pioneers of the new wind chill research, Randall Osczevski, discussed the New formula (which was actually developed in the 1990s) in this paper and uses values as low as -100°F with air temperatures as low as -60°F. He also discusses how the new formula is more realistic at extreme low temperatures. Also, since NOAA calculates wind chill values for any temperature and any wind speed (over 3 mph), we can assume an implicit endorsement of the formula at extreme low temperatures. Therefore, we consider the McGrath wind chill value of -99.8°F a valid wind chill record.

Summary

Circumstantial evidence supports the observations from the Howard Pass RAWS station during the February 2013 low wind chill event. We therefore consider the wind chill observation to be valid and consider the reading a statewide and national record – along with the aforementioned reading from McGrath in 1989. If the National Park Service is able to keep the station up and running for a number of years, we expect this record to be broken several times in the future.

Acknowledgements

The National Park Service generously provided the raw station data, site photographs, and instrument specifications for this analysis.

20 comments:

Very well done Brian. I was skeptical in the past, but after considering the graphic trend of wind and temps at Howard Pass, it now seems quite plausible.

The McGrath record would seem to be more a function of severely cold air than periodic wind. Unlike Howard Pass that appears to be chronically cold and windy, any brief breeze in cold locations could trigger an extreme wind chill.

What about elevated areas near Denali Park for example? Surely the summits are exposed to cold and wind. But even so, that does't diminish the unique data from Howard Pass.

This analysis would be worthy of a professional paper in support of the NPS' efforts to monitor climate. Here's link to their webpage well worth exploring:

Glad you enjoyed the post Gary. Without a doubt the -100 chill at Howard Pass felt quite different than he -100 chill at McGrath. Instead of a temperature value, perhaps a metric that calculates the number of minutes until death would be more appropriate.

Eric, the pre-2001 wind chill formula was based on calculations that measured how long it took for water to freeze. Therefore, it is probably a more accurate measurement to estimate the time to freeze to death. The McGrath wind chill using the old formula was *only* -93.8°F while the Howard Pass wind chill was -130.7°F using the old formula.

I'd take cold over wind any day. I suppose it's a matter of personal preference.

But surely humidity plays a role as well but is apparently left out of the current calculations...for example at the same air temp it feels to me to be colder when the humidity is higher. I like the old freezing water/wind chill method and keep a dated chart that reflects that determination.

In reverse, I guarantee that throwing water on heated rocks in a sauna creates an immediate appreciation for relative humidity.

At -70, there isn't much humidity left since most has frozen out. At -40, the air is 1000 times drier than at room temp. Imagine what it is at -70. But at 20 above, the humidity makes a big difference.

I wonder how warm the liquid in the "throw boiling water outside at -40" experiment would have to be at -70 to work.

I would think that the water temperature required for crystallization is 30° less at -70° than at -40°. Here is a good description of the physics behind the process: https://medium.com/starts-with-a-bang/why-can-hot-water-freeze-faster-than-cold-48b909661759

Ok, back to topic and a question. The sandblasting observation from might confirm my suspicion of damage from blown FOD to the sensors noted in an earlier Blog. I've seen that before near Delta Junction on communications towers sited on rocky terrain in habitually windy areas.

If the project were mine I'd explore some alternative siting less exposed to surface erosion. If that's not feasible (Caribou can safely be directed around rock cairns (inuksuit) or other temporary surface structures), then perhaps a taller support mast for the anemometer is in order. That would be to reduce the unit's exposure to the near-surface flow of debris if it's differential by elevation.

I'm aware of the expense and logistics involved in erecting and maintaining a tower. Product to site, labor and materials to set a tower base, then installing and guying the structure. Even a simple mast may reveal the critical elevation of the scouring effect and offer a clue as to how high the sensors need be. Two anemometers might be beneficial...one to drive the hourly reports and one strictly tasked to record wind between downloads.

Question. Is there any evidence on differential erosion from debris by height with the existing structure?

Gary, on Figure 3, the black discoloration behind the equipment box is from the sand-blasting of the back side of the solar panel.

It occurred to me that since the equipment is in a National Preserve (it is just outside of the Wilderness boundary), there may be National Park Service regulations. Here is the vision statement for Noatak National Preserve as stated in ANILCA Section 201:

"The preserve shall be managed for the following purposes, among others: To maintain the environmental integrity of the Noatak River and adjacent uplands within the preserve in such a manner as to assure the continuation of geological and biological processes unimpaired by adverse human activity; to protect habitat for, and populations of, fish and wildlife, including not limited to caribou, grizzly bears, Dall sheep, moose, wolves, and for waterfowl, raptors, and other species of birds; to protect archeological resources; and in a manner consistent with the foregoing, to provide opportunities for scientific research."

My guess is that they wanted to make sure that it was basically out of sight for all humans and ungulates.

I'm not going there much because the two are incongruous...witness the compromised station. The early inhabitants from Siberia didn't care (Caribou fences and Petroglyphs) yet today any human presence is constrained.

Those are great references Gary – as always. I can only imagine what the cost of maintaining a remote stations is. Unfortunately the wind instrumentation probably won't be repaired until July or August so we'll just have to wait until next winter to get a better picture of the wind climatology. Even if the wind were calm, it is probably impracticable to repair/replace any equipment due to the cold temperatures.

It's hard to know if the sensor was damaged by flying debris (sand, rocks, ice) or just cracked/ripped off by the cold temps and high winds. From looking at the enclosure box and solar panel, there's obvious evidence of sand/ice blasting. Slightly higher on the mast (2 m), the backup radiation shield was damaged in 2012 by either debris, rime, or wildlife. On the higher cross arms (3 m; where the wind sensor is mounted) the tipping bucket doesn't show any evidence of being pummeled by rocks or ice.

Blowing debris is an obvious suspect. However, I have seen the same sensor survive gusts over 100 mph at other (warmer) locations in scoured terrain with loose rocks and ice at the surface. The cold temperatures may have a lot to do with the problems at Howard Pass.

We are discussing many possibilities to keep the station running within logistical and bureaucratic constraints. Appreciate the interest and suggestions from this blog.

Hello Ken from Gary in Fairbanks. When the anemometer fails to respond what's generally compromised? Is it the sensor, mountings, cabling, or ???

The height comments of mine and yours regarding scouring appear to be a typical artifact of surface debris being hurled about...be it ice or terrestrial in origin.

Your program (and others in the NPS system) are important for documenting trends as noted in the mission statement for the projects. I hope it will continued to be supported financially and administratively.

At our summer 2014 field visit the mount and cabling for the wind sensor were all intact (as were all other sensors). The lowest section of the wind sensor was still mounted on the crossarm. Some remaining parts of the sensor were found in pieces (likely caused by impact with the ground) ~100 m downwind. I can send you more photos if you are interested. My contact is kenneth_hill@nps.gov